1. Field of the Invention
The present invention relates to a hammer drill and/or percussion hammer according to the preamble of patent claim 1.
2. Description of the Related Art
A hammer drill and/or percussion hammer, designated “hammer” in the following, standardly has an air pneumatic spring hammer mechanism in which a drive plunger is set into an oscillating back-and-forth movement by an electric motor, using a crankshaft or wobble shaft drive. A percussion piston is situated before the drive piston, so that a hollow space, in which an air spring can form, is present between the drive piston and the percussion piston. The air spring transmits the back and forth movement of the drive piston to the percussion piston, and drives this percussion piston against the shaft of a tool or against an intermediately connected rivet header. Hammers of this sort are known in many different specific embodiments.
In the use of the hammer for work at a particular location, the operator must place the tip of the tool, for example the chisel tip, very carefully in order to prevent the chisel tip from jumping away. This is true in particular for relatively smooth or raised points on the material to be worked. However, because air spring hammer mechanisms having a simple design have a tendency to begin percussion operation suddenly, the undesired jumping away cannot always be avoided. This can have the result that the stone is chiseled at a point that is not supposed to be damaged. In the processing of edges, there is even the danger of damage to the hammer or to the operator himself, if the chisel jumps into the air from the edge.
In order to remove this problem, various solutions have been proposed. Thus, it is known to avoid, or at least to attenuate, an abrupt beginning of percussive operation by reducing the no-load motor speed. However, this has the disadvantage that the characteristic of the motor speed is always the same when accelerating from no-load operation to percussion operation, while the particular case of application requires a specifically adapted run-up. In addition, the lowering of the motor speed prevents the rapid creation of a stabilizing centering in the material that is to be worked.
A different solution is described for example in DE-A-197 13 154 or in DE-A-197 24 531, in the form of what is known as a sleeve controlling. Here, the effect is exploited that the tool is held so as to be capable of axial motion relative to the hammer, and can slide out of the hammer housing somewhat in the no-load setting. When the tool is placed on the stone that is to be worked, the shaft of the tool is pushed into the interior of the hammer, and effects (standardly by displacing the percussion piston relative to the drive piston) a transition from no-load operation to percussion operation.
In sleeve controlling, the relative movement of the tool to the hammer housing is transmitted to a spring-loaded control sleeve either directly or via an intermediate piston. The control sleeve works together with control bores, with which a no-load air channel can be opened and closed that connects the hollow space that accommodates the air spring, situated between the drive and percussion piston, with the surrounding environment. The displacement of the control sleeve thus makes it possible to bring the hollow space into communicating connection with the surroundings of the and are mechanism, or to close such a connection. Through this controlling of the ventilation of the air spring hollow space, the change between no-load operation and percussion operation can be realized very reliably.
Because in sleeve controlling the motor speed of the drive motor, and thus the number of impacts, remain almost unchanged, a centering in the material being processed, providing a place of purchase for the tool, can be produced very quickly, in contrast to the above-described reduction of motor speed. Through this high degree of controllability, the operator can optimally determine the strength of the individual impacts for each particular case of application.
However, sleeve controlling also has a disadvantage. As already explained, during the movement of the tool shaft into the interior of the hammer housing, the control sleeve is displaced against the action of a spring. Thus, the pressure to be applied by the operator is increased by the spring force between the tool shaft, or a rivet header connected thereto, and the hammer housing. Especially in heavy hammers, this is disadvantageous because the spring acting on the control sleeve must be designed such that it has to support at least the weight of the tool on the one hand, or the weight of the hammer on the other hand, in order to avoid an undesired change from no-load operation to percussion operation. If work is to be carried out with the hammer oriented upward, this means that even in no-load operation the entire weight of the tool lies against the control sleeve, and thus against the spring, so that the spring has to hold the tool. The change to percussion operation must take place only when the tool is pressed against the stone that is to be worked.
The same holds for work oriented downward. Here, in particular for heavy breaking hammers it must be possible to place the tool on the ground and to support the entire hammer on the tool, while remaining in no-load operation. Percussion operation should begin only when the hammer is pressed downward by the operator.
Given a change of position of the hammer, for example given operation in the horizontal direction, the support due to the weight of the tool or hammer, otherwise present, is in addition missing. The operator then must apply still greater forces.